1. Field of the Invention
The present invention finds principal application within the field of marine seismic exploration. More particularly, the invention is concerned with means for accurately determining the position of a towed marine seismic streamer.
2. Prior Art
In marine seismic prospecting, an exploration vessel tows a seismic streamer having a plurality of pressure sensitive detectors, commonly referred to as hydrophones. A source of seismic energy, such as an air gun or an explosive charge, is used to propagate pressure waves through the water into the underlying sea floor. Part of the energy will be reflected by subfloor geological discontinuities and subsequently detected by the hydrophones as pressure variations in the surrounding water. The mechanical energy of these pressure variations is transformed into an electrical signal by the hydrophones and transmitted through the streamer to a recording apparatus aboard the vessel. The collected data may then be interpreted by those skilled in the art to reveal information about the subsea geological formations. Modern methods of seismic data collection use redundant subsurface coverage by successive source-hydrophone positions. This allows the data to be stacked to enhance the signal to noise ratio.
For the signals to be meaningful and to permit correct stacking of signals from different source and hydrophone positions, it is necessary to known the placement of the individual hydrophones at the time the pressure waves are detected. As the vessel is continuously moving and as the streamer may extend for thousands of feet behind the vessel, accurate location of the streamer hydrophones is difficult.
Various systems have been developed to provide accurate information as to the location of the vessel. The most common means of identifying the position of the streamer is by satellite and radio navigation fixes on the vessel, and then some estimation of the streamer location. Another less common method involves the use of bottom founded ocean transponders to be interrogated by streamer bound transceivers. Here, the vessel's position with respect to the transponders may be triangulated if the location of the transponders are known.
However, it is rare for the streamer to trail directly along the path of the vessel. While the streamer is attached to the stern of the vessel, the bulk of the streamer is submerged below the water surface through the action of depth controllers along the length of the streamer. As a result, the cross-track current velocity at the streamer depth may differ from the cross-track current affecting the vessel, thereby causing the streamer to trail at an angle to the vessel's course. Other factors, which are not necessary to enumerate, may also create a variance in the path of the streamer when compared to the vessel track.
One method of estimating the location of the streamer disclosed in the prior art relies upon the addition of a tail buoy radar reflector located at the end of the streamer. On-board radar systems may then be used under optimal sea conditions to find the end of the streamer and the location of the individual hydrophones interpolated. Such system are generally unreliable however, and render the required data suspect.
A second method taught by the art relies upon very sensitive and expensive apparatus to measure the yaw and pitch angles of the streamer end adjacent the vessel. See Rice Jr. et al, U.S. Pat. No. 4,068,208. These data, coupled with magnetic compass headings taken along the streamer and the known depth of the streamer, permits one to empirically calculate the hydrophone locations.
Another method may use an acoustic source mounted on the vessel. This source is used to interrogate hydrophones located in the streamer to determine their distance from the vessel. See Brown, U.S. Pat. No. 3,840,845; Itria, et al, U.S. Pat. Nos. 4,063,213 and 4,087,780; and Roberts, U.S. Pat. No. 4,376,301.
There are also methods that employ two boats to simultaneously transmit acoustic pulses to locate a submerged marine streamer. However, the basic problem with these types is their lack of acoustic range in the surface layer of the ocean due to the high attenutation caused by entrapped gases and the inherent negative velocity character of this layer. Under certain conditions sound waves are deflected downward away from surface level equipment, which means that the effort to locate the position of the streamer will be hampered.